During earthen drilling operations using sealed bearing drill bits, such as rotary cone drill bits, it is necessary to protect the bearing elements of the bit from contamination in order to sustain bit operability. In particular, it is desirable to isolate and protect the bearing elements of the bit, such as bearings, bearing lubricant and bearing surfaces that are located in a bearing cavity or cavities between each corresponding bit leg and roller cone, from earthen cuttings, mud and other debris in the drilling environment. The introduction of such contaminants into the bearing system of the drill bit can lead to deterioration of the bearing lubricant, bearings and bearing surfaces, resulting in premature bit failure. An annular seal is, therefore, placed in the bit between the external environment and the bearing to prevent such unwanted contaminants from entering the drill bit through the annular opening and into a gap formed between each leg and corresponding roller cone that extends to the bearing cavity.
In a downhole drilling environment, the borehole contains “drilling fluid,” which can be drilling mud, other liquids, air, other gases, or a mixture or combination thereof. In a typical liquid drilling environment of a petroleum well, the downhole fluid pressure at the location of the drill bit, i.e., the “external pressure,” can be very high and fluctuating. At the same time, internal pressure within the bearing cavity, i.e., the “internal pressure,” can also be very high and fluctuating due, for example, to thermal expansion and out-gassing of lubricant in the bearing cavity, and to cone movement relative to the leg. These high pressure changes internal and external to the bearing cavity may cause a differential pressure across the annular seal, thus resulting in a major unchecked load on the seal.
When the internal pressure is greater than the external pressure, the seal may be drawn to and possibly extruded into the gap. Likewise, a greater external pressure can cause the seal to be drawn in the direction of the bearing cavity and possibly extruded therein. This may cause excessive wear to or tearing of the seal, which can eventually lead to bit inoperability. Furthermore, when the pressure differential between the bit internal and external environments reaches a certain level in each above scenario, the seal can leak, allowing lubricant to pass from the bearing cavity into the gap in the first scenario, and drilling fluid to pass from the gap into the bearing cavity in the second scenario.
Generally, when the internal pressure and the external pressure are equal, the differential pressure across the bearing cavity seal will be zero. There will be no pressure to force the drilling fluid or lubricant by the seal, or to force the seal into the gap or bearing cavity. Thus, it is generally desirable to achieve or maintain a differential pressure of approximately zero across the bit during operation. Drill bits are, therefore, constructed having a lubricant reservoir system disposed therein to equalize the internal and external pressure across the seal. Such lubricant reservoir systems typically have a flexible diaphragm located in a lubricant reservoir cavity placed in the bit leg. The flexible diaphragm operates to separate the internal lubricant from the external drilling fluid and communicates the external pressure to the portion of the bearing seal adjacent the bearing cavity. This type of pressure compensation system for a single seal bit is schematically shown in FIG. 1A.
Referring to FIG. 1A, when the external or borehole pressure Pd of the drilling fluid in the borehole B1 increases and is greater than the internal pressure Pg in the bearing cavity, the seal S1 will be forced inwardly toward the bearing cavity B2. With the use of a flexible diaphragm D1, the external pressure Pd is also applied to the diaphragm D1, which transmits the pressure Pd, equalizing it with the internal pressure Pg. As a result, the pressure on both sides of the seal S1 is balanced, preventing the occurrence of any differential pressure across the seal S1. Similarly, when the pressure Pg increases, Pg will temporarily be larger than Pd, causing the diaphragm D1 to expand outwardly to increase the internal volume of the bearing cavity B2. As the internal volume increases, the internal pressure Pg will decrease. Pg will drop to equilibrium with Pd, and the internal volume will stop increasing.
Dual seal arrangements have been proposed having an outer seal positioned within a seal gland located between the external environment and a primary inner seal. The purpose of including a second seal is typically to provide a second layer or barrier of protection from particles entering the gap through the annular opening. When an outer seal is added, it may be necessary, such as in drill bits used for petroleum wells, that the bit be capable of compensating for the differential pressure across both seals. FIG. 1B shows a dual-seal bit schematic with both seals providing substantially absolute seals. The “space” Sp formed between the seals S1, S2 is completely filled with an incompressible fluid, and there is no variation in the density of the incompressible fluid.
In this scenario, the incompressible fluid in space Sp between the seals S1 and S2 transmits pressure from Pg1, which is the (internal) bearing cavity pressure, to Pd and from Pd to Pg1. For example, when the external fluid pressure Pd increases, the diaphragm D1 will be pushed inwardly, causing the internal pressure Pg1 to equal the external pressure Pd. Because the fluid between seals S1 and S2 is incompressible, it will transmit the increased pressure between S1 and S2, and neither seal S1 nor S2 will be displaced.
However, during borehole drilling operations, such as with rotary cone sealed bearing drill bits, various factors will alter ideal conditions and require something more to equalize the differential pressure across both seals S1 and S2. For example, there can be a relative movement between the roller cone and bit leg, which causes the volume of the space Sp between the seals S1 and S2 to significantly increase and decrease. A change in the volume of the space Sp will change the chamber pressure Pg2 in the space Sp, causing conditions where Pg2>Pd, Pg1 upon contraction of the space Sp, and where Pg2<Pd, Pg1 upon expansion of the space Sp. Thus, there can be differential pressures across both seals S1, S2, causing their movement and possible extrusion, which can cause accelerated seal wear and eventual bit failure.
Another potential factor altering ideal conditions is the thermal expansion, or out-gassing, of the incompressible fluid between the seals S1, S2 due to elevated temperatures within the bit. Referring to FIG. 1B, expansion of the incompressible fluid in the space Sp between the seals S1, S2 will elevate the chamber pressure Pg2. Increasing the chamber pressure Pg2 can cause a differential pressure across the seals S1, S2 such that Pg2>Pd, Pg1, which can result in accelerated wear and possible extrusion of seals S1, S2.
Still another factor is the existence of air trapped in the space Sp between the seals S1, S2. In this instance, the mixture of air and fluid in space Sp is not incompressible. When external pressure Pd increases, Pg1 will eventually equal Pd due to the diaphragm D1, but Pd>Pg2 and Pg1>Pg2 because of the presence of air in the space Sp between the seals S1, S2. The chamber pressure Pg2 in the space Sp will not increase until the seals S1, S2 move closer together and the air volume in space Sp decreases. This differential pressure across seals S1, S2 will cause the movement and possible extrusion of the seals into the space Sp and excessive wear on the seals.
U.S. Pat. No. 5,441,120, which is hereby incorporated by reference herein in its entirety, discloses the use of an additional flexible diaphragm D2, such as that shown in FIG. 1C, to attempt to equalize, or balance the chamber pressure Pg2 of the space Sp with the external pressure Pd or internal pressure Pg1. Further increases in external pressure Pd will thereafter be transmitted through the fluid in the space Sp. Such a system has various disadvantages. For example, this system requires or occupies much space within the bit leg, structurally weakening the bit, and limiting the size of bits that can incorporate such system. Also, this system does not allow for pressure relief from the space Sp, such as caused by thermal expansion and outgassing of the incompressible fluid between the seals S1, S2, which can cause damage to the seals as described above.
U.S. Pat. Nos. 4,981,182 and 5,027,911, which are also hereby incorporated herein in their entireties, disclose various embodiments of drill bits having inner and outer seals where the lubricant is bled out of the bit past the outer seal to prevent drilling debris from accumulating and damaging the inner and outer seals. In some such embodiments, passages in the bit allow lubricant to travel from the bearing cavity to the space between the seals. In other embodiments, a hydrodynamic inner seal is used, which allows the leakage of lubricant from the bearing cavity to the space between the seals. In both instances, the pressure of the lubricant presumably forces the outer seal to open and allow the bleeding of lubricant from the bit.
These systems also have various disadvantages. For example, the continuous bleeding of lubricant past the outer seal (particularly if the outer seal fails) can lead to the depletion of bearing lubricant in the bit, and cause bearing and bit damage due to a lack of lubricant. For another example, if the space between the seals in such configurations is not filled with lubricant, which will occur if there is a decrease or stoppage in the flow of lubricant from the bearing cavity to the space, a high pressure differential across the seals can result, causing damage to the seals as described above. For yet another example, with many such embodiments, because the space between the seals and the bearing cavity are in fluid communication, there exists the possibility that debris or drilling fluid bypassing the outer seal, such as when the outer seal fails, will move through the space between the seals and into the bearing cavity, causing contamination and damage to therein and to the bearing elements.
Therefore, there remains a need for improved techniques and mechanisms for substantially balancing or minimizing the pressure differential imposed upon either a single seal within a drill bit, or upon primary and secondary seals of a dual-seal configuration, particularly by allowing pressure communication and for equalization between the interior and exterior of the drill bit. Ideally, the devices and techniques will accommodate cone movement, thermal expansion of the fluid and/or out-gassing between the primary and secondary seals, and trapped air in the space between the seals. It is also desired that such pressure communication devices that do not require substantial additional components, large space requirements in the bit, or highly complex manufacturing requirements.
Also well received would be a pressure communication technique and device capable of preventing the pressure differential across the dual seals from exceeding an upper limit, such as, for example, 100 psi. It would also be advantageous to include the use of an incompressible fluid having the capabilities of retaining sufficient viscosity to act as a medium for the transmission of energy between the primary and secondary seals, of retaining its lubrication properties, and/or of slowing the intrusion of abrasive particles to the primary seal when and after the incompressible fluid is exposed to drilling fluid.